1. Field of the Invention
The present invention relates to a quadrature demodulator and an interrogator equipped with the quadrature demodulator for, when a reception signal is demodulated, creating an I signal and a Q signal of a baseband.
2. Description of the Related Art
An interrogator is a radio communication device that makes radio communication with a transponder called a radio tag or RFID (radio frequency identification) tag. The interrogator transmits data to the RFID tag by using a modulated radio signal, and then, continuously transmits a non-modulated signal after the end of transmitting the data. In contrast, the RFID tag carries out backscatter modulation that changes a reflection amount of the non-modulated signal from the interrogator to transmit data to the interrogator. The interrogator receives a backscatter modulation wave, and then, reads data on the RFID tag.
The interrogator is equipped with a transmitting section and a receiving section. At the transmitter side, data is subjected to modulation by a modulator, amplified by an amplifier, and then transmitted from an antenna. At the receiver side, from a high frequency signal which is the signal received by an antenna, a baseband signal is extracted by a direct conversion quadrature demodulator, and then demodulated to derive data.
The direct conversion quadrature demodulator inputs to a mixer a local signal and a reception signal having a frequency that is equal to that of a carrier of the reception signal to create an I (in-phase) signal of a baseband, and then, inputs to the mixer a signal shifted in phase by 90 degrees with respect to the local signal and the reception signal to create a Q (quadrature-phase) signal of a baseband.
Amplitudes of the I signal and Q signal are determined depending on a phase difference between the reception signal and the local signal. When the amplitude of the I signal becomes maximal, the amplitude of the Q signal becomes minimal, and when the amplitude of the I signal becomes minimal, the amplitude of the Q signal becomes maximal. When the amplitude of the Q signal is 0 that is minimal, the amplitude of the I signal is maximal. Thus, reception data can be reproduced using this I signal. Conversely, when the amplitude of the I signal is 0 that is minimal, the amplitude of the Q signal is maximal. Thus, reception data can be reproduced using this Q signal. In addition, the phases of the I signal and the Q signal may be inverted depending on a phase difference between the reception signal and the local signal.
As a method for reproducing reception data by using the direct conversion quadrature demodulator described above, there is known a method for comparing the amplitudes of the I signal and the Q signal with each other, selecting one of the signals that has a greater amplitude, and then, reproducing reception data (reference should be made to U.S. Pat. No. 6,501,807 B1, for example).
The reproducing method described in U.S. Pat. No. 6,501,807 B1 compares the amplitudes of the I signal and the Q signal with each other, selects one of the signals that has a greater amplitude, and reproduces reception data. Thus, when the amplitudes of the I signal and the Q signal are greatly different from each other, the amplitude of the selected signal is also great, and therefore, no problem occurs with reproduction. However, in the case where the amplitudes of the I signal and the Q signal are substantially equal to each other, reception data must be reproduced at an amplitude that is half of that of the reception signal, although either of the amplitudes may be selected. Thus, there has been a problem that when a level of the reception signal is low, the reception signal is easily influenced by noises, and then, incorrect reproduction of the reception data due to noises frequently occurs.
Therefore, the present invention is directed to the provision of a quadrature demodulator and an interrogator equipped with the quadrature demodulator, which can reliably suppress the influence of noises even if the level of the reception signal becomes low, thereby reducing an occurrence of incorrect reproduction of the reception data due to noises.